Decadal climate sensitivity of contouritic sedimentation in a dynamically coupled ice-ocean-sediment model of the Pan-Arctic region

2021 ◽  
Author(s):  
Catherine Drinkorn ◽  
Jan Saynisch-Wagner ◽  
Gabriele Uenzelmann-Neben ◽  
Maik Thomas
Keyword(s):  
Deep Water ◽  
Arctic Region ◽  
Meridional Overturning Circulation ◽  
Ocean Dynamics ◽  
Sedimentation Rates ◽  
Climate Conditions ◽  
The North ◽  
Ocean Sediment ◽  
Set Up ◽  
Sediment Model

<p>Ocean sediment drifts contain important information about past bottom currents but a direct link from the study of sedimentary archives to ocean dynamics is not always possible. To close this gap for the North Atlantic, we set up a  new coupled Ice-Ocean-Sediment Model of the entire Pan-Arctic region. In order to evaluate the potential dynamics of the model, we conducted decadal sensitivity experiments. In our model contouritic sedimentation shows a significant sensitivity towards climate variability for most of the contourite drift locations in the model domain. We observe a general decrease of sedimentation rates during warm conditions with decreasing atmospheric and oceanic gradients and an extensive increase of sedimentation rates during cold conditions with respective increased gradients. We can relate these results to changes in the dominant bottom circulation supplying deep water masses to the contourite sites under different climate conditions. A better understanding of northern deep water pathways in the Atlantic Meridional Overturning Circulation (AMOC) is crucial for evaluating possible consequences of climate change in the ocean.</p>

scholarly journals Fingerprints of changes in the terrestrial carbon cycle in response to large reorganizations in ocean circulation

2010 ◽  
Vol 6 (5) ◽  
pp. 1811-1852 ◽  
Author(s):  
A. Bozbiyik ◽  
M. Steinacher ◽  
F. Joos ◽  
T. F. Stocker
Keyword(s):  
Carbon Cycle ◽  
Ocean Circulation ◽  
North Atlantic ◽  
Climate Model ◽  
Carbon Stocks ◽  
Meridional Overturning Circulation ◽  
Climate Conditions ◽  
Tropical Regions ◽  
The North ◽  
Northern Latitudes

Abstract. CO2 and carbon cycle changes in the land, ocean and atmosphere are investigated using the comprehensive carbon cycle-climate model NCAR CSM1.4-carbon. Ensemble simulations are forced with freshwater perturbations applied at the North Atlantic and Southern Ocean deep water formation sites under pre-industrial climate conditions. As a result, the Atlantic Meridional Overturning Circulation reduces in each experiment to varying degrees. The physical climate fields show changes that are well documented in the literature but there is a clear distinction between northern and southern perturbations. Changes in the physical variables affect, in return, the land and ocean biogeochemical cycles and cause a reduction, or an increase, in the atmospheric CO2 by up to 20 ppmv, depending on the location of the perturbation. In the case of a North Atlantic perturbation, the land biosphere reacts with a strong reduction in carbon stocks in some tropical locations and in high northern latitudes. In contrast, land carbon stocks tend to increase in response to a southern perturbation. The ocean is generally a sink of carbon although large re-organizations occur throughout various basins. The response of the land biosphere is strongest in the tropical regions due to a shift of the Intertropical Convergence Zone. The carbon fingerprints of this shift, either to the south or to the north depending on where the freshwater is applied, can be found most clearly in South America. For this reason, a compilation of various paleoclimate proxy records of Younger Dryas precipitation changes are compared with our model results.

Identification and quantification of the North Atlantic Deep Water pathways

2021 ◽  
Author(s):  
Philippe Miron ◽  
Maria J. Olascoaga ◽  
Francisco J. Beron-Vera ◽  
Kimberly L. Drouin ◽  
M. Susan Lozier
Keyword(s):  
North Atlantic ◽  
Deep Water ◽  
Sea Water ◽  
Lower Layer ◽  
North Atlantic Deep Water ◽  
Meridional Overturning Circulation ◽  
Labrador Sea ◽  
Western Boundary ◽  
The North ◽  
The North Atlantic

<p>The North Atlantic Deep Water (NADW) flows equatorward along the Deep Western Boundary Current (DWBC) as well as interior pathways and is a critical part of the Atlantic Meridional Overturning Circulation. Its upper layer, the Labrador Sea Water (LSW), is formed by open-ocean deep convection in the Labrador and Irminger Seas while its lower layers, the Iceland–Scotland Overflow Water (ISOW) and the Denmark Strait Overflow Water (DSOW), are formed north of the Greenland–Iceland–Scotland Ridge.</p><p>In recent years, more than two hundred acoustically-tracked subsurface floats have been deployed in the deep waters of the North Atlantic.  Studies to date have highlighted water mass pathways from launch locations, but due to limited float trajectory lengths, these studies have been unable to identify pathways connecting  remote regions.</p><p>This work presents a framework to explore deep water pathways from their respective sources in the North Atlantic using Markov Chain (MC) modeling and Transition Path Theory (TPT). Using observational trajectories released as part of OSNAP and the Argo projects, we constructed two MCs that approximate the lower and upper layers of the NADW Lagrangian dynamics. The reactive NADW pathways—directly connecting NADW sources with a target at 53°N—are obtained from these MCs using TPT.</p><p>Preliminary results show that twenty percent more pathways of the upper layer(LSW) reach the ocean interior compared to  the lower layer (ISOW, DSOW), which mostly flows along the DWBC in the subpolar North Atlantic. Also identified are the Labrador Sea recirculation pathways to the Irminger Sea and the direct connections from the Reykjanes Ridge to the eastern flank of the Mid–Atlantic Ridge, both previously observed. Furthermore, we quantified the eastern spread of the LSW to the area surrounding the Charlie–Gibbs Fracture Zone and compared it with previous analysis. Finally, the residence time of the upper and lower layers are assessed and compared to previous observations.</p>

Does ocean resolution affect the rate of AMOC weakening?

2020 ◽  
Author(s):  
Helene Hewitt ◽  
Laura Jackson ◽  
Malcolm Roberts ◽  
Dorotea Iovino ◽  
Torben Koenigk ◽  
...  
Keyword(s):  
Deep Water ◽  
Horizontal Resolution ◽  
Meridional Overturning Circulation ◽  
Subpolar Gyre ◽  
Deep Water Formation ◽  
North Atlantic Current ◽  
The North ◽  
Water Formation ◽  
The Mean ◽  
Mean State

<p>We examine the weakening of the Atlantic Meridional Overturning Circulation (AMOC) in response to increasing CO<sub>2</sub> at different horizontal resolutions in HadGEM3-GC3.1 and in a small ensemble of models with differing resolutions. There is a strong influence of the ocean mean state on the AMOC weakening: models with a more saline western subpolar gyre have a greater formation of deep water there. This makes the AMOC more susceptible to weakening from an increase in CO<sub>2</sub> since weakening ocean heat transports weaken the contrast between ocean and atmospheric temperatures and hence weaken the buoyancy loss. In models with a greater proportion of deep water formation further north (in the Greenland-Iceland-Norwegian basin), deep-water formation can be maintained by shifting further north to where there is a greater ocean-atmosphere temperature contrast.</p><p>We show that ocean horizontal resolution can have an impact on the mean state, and hence AMOC weakening. In the models examined, those with higher resolutions tend to have a more westerly path of the North Atlantic Current and hence greater impact of the warm, saline subtropical Atlantic waters on the western subpolar gyre. This results in greater dense water formation in the western subpolar gyre. Although there is some improvement of the higher resolution models over the lower resolution models in terms of the mean state, both still have biases and it is not clear which biases are the most important for influencing the AMOC strength and response to increasing CO<sub>2</sub>.</p><p> </p>

scholarly journals Fingerprints of changes in the terrestrial carbon cycle in response to large reorganizations in ocean circulation

2011 ◽  
Vol 7 (1) ◽  
pp. 319-338 ◽  
Author(s):  
A. Bozbiyik ◽  
M. Steinacher ◽  
F. Joos ◽  
T. F. Stocker ◽  
L. Menviel
Keyword(s):  
Carbon Cycle ◽  
Ocean Circulation ◽  
North Atlantic ◽  
Climate Model ◽  
Carbon Stocks ◽  
Meridional Overturning Circulation ◽  
Climate Conditions ◽  
Tropical Regions ◽  
Proxy Records ◽  
The North

Abstract. CO2 and carbon cycle changes in the land, ocean and atmosphere are investigated using the comprehensive carbon cycle-climate model NCAR CSM1.4-carbon. Ensemble simulations are forced with freshwater perturbations applied at the North Atlantic and Southern Ocean deep water formation sites under pre-industrial climate conditions. As a result, the Atlantic Meridional Overturning Circulation reduces in each experiment to varying degrees. The physical climate fields show changes qualitatively in agreement with results documented in the literature, but there is a clear distinction between northern and southern perturbations. Changes in the physical variables, in turn, affect the land and ocean biogeochemical cycles and cause a reduction, or an increase, in the atmospheric CO2 concentration by up to 20 ppmv, depending on the location of the perturbation. In the case of a North Atlantic perturbation, the land biosphere reacts with a strong reduction in carbon stocks in some tropical locations and in high northern latitudes. In contrast, land carbon stocks tend to increase in response to a southern perturbation. The ocean is generally a sink of carbon although large reorganizations occur throughout various basins. The response of the land biosphere is strongest in the tropical regions due to a shift of the Intertropical Convergence Zone. The carbon fingerprints of this shift, either to the south or to the north depending on where the freshwater is applied, can be found most clearly in South America. For this reason, a compilation of various paleoclimate proxy records of Younger Dryas precipitation changes are compared with our model results. The proxy records, in general, show good agreement with the model's response to a North Atlantic freshwater perturbation.

scholarly journals Millennial-scale variations of sedimentary oxygenation in the western subtropical North Pacific and its links to the North Atlantic climate

2019 ◽  
Author(s):  
Jianjun Zou ◽  
Xuefa Shi ◽  
Aimei Zhu ◽  
Selvaraj Kandasamy ◽  
Xun Gong ◽  
...  
Keyword(s):  
North Atlantic ◽  
Deep Water ◽  
North Pacific ◽  
Co2 Concentration ◽  
Meridional Overturning Circulation ◽  
Atmospheric Co2 ◽  
Intermediate Water ◽  
The North ◽  
The North Atlantic ◽  
Millennial Scale

Abstract. Lower glacial atmospheric CO2 concentrations have been attributed to carbon sequestration in deep oceans. However, potential roles of voluminous subtropical North Pacific in modulating atmospheric CO2 levels on millennial timescale are poorly constrained. Further, an increase in respired CO2 concentration in the glacial deep ocean due to biological pump generally is coeval with less oxygenation in the subsurface layer. This link thus offers a chance to visit oceanic ventilation and the coeval export productivity based on redox-controlled sedimentary geochemical parameters. Here we investigate a suite of sediment geochemical proxies to understand the sedimentary oxygenation variations in the subtropical North Pacific (core CSH1) over the last 50 thousand years (ka). Our results suggest that sedimentary oxygenation at mid-depths of the subtropical North Pacific intensifies during the episodes of late glacial (50–25 ka), Last Glacial Maximum (LGM) and also the interval after 8.5 ka, especially pronounced for the North Atlantic millennial-scale abrupt cold events of the Younger Dryas, Heinrich Stadial (HS) 1 and 2. On the other hand, oxygen-depleted seawater is found during the Bölling-Alleröd (B/A) and Preboreal. Our findings of enhanced sedimentary oxygenation in the subtropical North Pacific is aligned with intensified formation of North Pacific Intermediate Water (NPIW) during cold spells, while the ameliorated sedimentary oxygenation seems to be linked with the intensified Kuroshio Current since 8.5 ka. In our results, diminished sedimentary oxygenation during the B/A indicates an enhanced CO2 sequestration at mid-depth waters, along with slight increase in atmospheric CO2 concentration. Mechanistically, we speculate that these millennial-scale changes were linked to the strength of North Atlantic Deep Water, leading to intensification of NPIW formation and enhanced abyss flushing during deglacial cold and warm intervals, respectively. Enhanced formation of NPIW seem to be driven by the perturbation of sea ice formation and sea surface salinity oscillation in high latitude North Pacific through atmospheric and oceanic teleconnection. During the B/A, decreased sedimentary oxygenation likely resulted from an upward penetration of aged deep water into the intermediate-depth in the North Pacific, corresponding to a resumption of Atlantic Meridional Overturning Circulation.

scholarly journals Changes in the geometry and strength of the Atlantic Meridional Overturning Circulation during the last glacial (20–50 ka)

2016 ◽  
Author(s):  
Pierre Burckel ◽  
Claire Waelbroeck ◽  
Yiming Luo ◽  
Didier Roche ◽  
Sylvain Pichat ◽  
...  
Keyword(s):  
Flow Rate ◽  
North Atlantic ◽  
Deep Water ◽  
Water Mass ◽  
Atlantic Meridional Overturning Circulation ◽  
Meridional Overturning Circulation ◽  
Overturning Circulation ◽  
The North ◽  
Meridional Overturning ◽  
The North Atlantic

Abstract. We reconstruct the geometry and strength of the Atlantic Meridional Overturning Circulation during Heinrich Stadial 2 and three Greenland interstadials of the 20–50 ka period based on the comparison of new and published sedimentary 231Pa/230Th data with simulated sedimentary 231Pa/230Th. We show that the deep Atlantic circulation during these interstadials was very different from that of the Holocene. Northern-sourced waters likely circulated above 2500 m depth, with a flow rate lower than that of the present day North Atlantic Deep Water (NADW). Southern-sourced deep waters most probably flowed northwards below 4000 m depth into the North Atlantic basin, and then southwards as a return flow between 2500 and 4000 m depth. The flow rate of this southern-sourced deep water was likely larger than that of the modern Antarctic Bottom Water (AABW). At the onset of Heinrich Stadial 2, the structure of the AMOC significantly changed. The deep Atlantic was probably directly affected by a southern sourced water mass below 2500 m depth, while a slow southward flowing water mass originating from the North Atlantic likely influenced depths between 1500 and 2500 m down to the equator.

North Atlantic Ocean Circulation Response to Stochastic Mesoscale Weather Systems

2021 ◽  
Author(s):  
Shenjie Zhou ◽  
Xiaoming Zhai ◽  
Ian Renfrew
Keyword(s):  
Ocean Circulation ◽  
North Atlantic ◽  
Climate Models ◽  
Meridional Overturning Circulation ◽  
Ocean Dynamics ◽  
Ensemble Prediction ◽  
Subpolar Gyre ◽  
Atmospheric Forcing ◽  
The North ◽  
The North Atlantic

<p>The ocean is forced by the atmosphere on a range of spatial and temporal scales. In ocean and climate models the resolution of the atmospheric forcing sets a limit on the scales that are represented. For typical climate models this means mesoscale (< 400 km) atmospheric forcing is absent. Previous studies have demonstrated that mesoscale forcing significantly affects key ocean circulation systems such as the North Atlantic Subpolar gyre and the Atlantic Meridional Overturning Circulation (AMOC). However, the approach of these studies has either been ad hoc or limited in resolution. Here we present ocean model simulations with and without realistic mesoscale atmospheric forcing that represents scales down to 10 km. We use a novel stochastic parameterization – based on a cellular automaton algorithm that is common in weather forecasting ensemble prediction systems<sup> </sup>– to represent spatially coherent weather systems over a range of scales, including down to the smallest resolvable by the ocean grid. The parameterization is calibrated spatially and temporally using marine wind observations. The addition of mesoscale atmospheric forcing leads to coherent patterns of change in the sea surface temperature and mixed-layer depth. It also leads to non-negligible changes in the volume transport in the North Atlantic subtropical gyre (STG) and subpolar gyre (SPG) and in the AMOC. A non-systematic basin-scale circulation response to the mesoscale wind perturbation emerges – an in-phase oscillation in northward heat transport across the gyre boundary, partly driven by the constantly enhanced STG, correspoding to an oscillatory behaviour in SPG and AMOC indices with a typical time scale of 5-year, revealing the importance of ocean dynamics in generating non-local ocean response to the stochastic mesoscale atmospheric forcing. Atmospheric convection-permitting regional climate simulations predict changes in the intensity and frequency of mesoscale weather systems this century, so representing these systems in coupled climate models could bring higher fidelity in future climate projections.</p>

scholarly journals Hydrological change in Southern Europe responding to increasing North Atlantic overturning during Greenland Stadial 1

2015 ◽  
Vol 112 (21) ◽  
pp. 6568-6572 ◽  
Author(s):  
Miguel Bartolomé ◽  
Ana Moreno ◽  
Carlos Sancho ◽  
Heather M. Stoll ◽  
Isabel Cacho ◽  
...  
Keyword(s):  
North Atlantic ◽  
Environmental Changes ◽  
Meridional Overturning Circulation ◽  
Ocean Dynamics ◽  
Gradual Transition ◽  
The North ◽  
Cave Monitoring ◽  
Meridional Overturning ◽  
The North Atlantic ◽  
Atmospheric Changes

Greenland Stadial 1 (GS-1) was the last of a long series of severe cooling episodes in the Northern Hemisphere during the last glacial period. Numerous North Atlantic and European records reveal the intense environmental impact of that stadial, whose origin is attributed to an intense weakening of the Atlantic Meridional Overturning Circulation in response to freshening of the North Atlantic. Recent high-resolution studies of European lakes revealed a mid–GS-1 transition in the climatic regimes. The geographical extension of such atmospheric changes and their potential coupling with ocean dynamics still remains unclear. Here we use a subdecadally resolved stalagmite record from the Northern Iberian Peninsula to further investigate the timing and forcing of this transition. A solid interpretation of the environmental changes detected in this new, accurately dated, stalagmite record is based on a parallel cave monitoring exercise. This record reveals a gradual transition from dry to wet conditions starting at 12,500 y before 2000 A.D. in parallel to a progressive warming of the subtropical Atlantic Ocean. The observed atmospheric changes are proposed to be led by a progressive resumption of the North Atlantic convection and highlight the complex regional signature of GS-1, very distinctive from previous stadial events.

scholarly journals Does Net E − P Set a Preference for North Atlantic Sinking?

2012 ◽  
Vol 42 (11) ◽  
pp. 1781-1792 ◽  
Author(s):  
Selma E. Huisman ◽  
Henk A. Dijkstra ◽  
A. S. von der Heydt ◽  
W. P. M. de Ruijter
Keyword(s):  
North Atlantic ◽  
Deep Water ◽  
North Pacific ◽  
Multiple Equilibria ◽  
Meridional Overturning Circulation ◽  
Global Ocean ◽  
Overturning Circulation ◽  
The North ◽  
Meridional Overturning ◽  
The North Pacific

Abstract The present-day global meridional overturning circulation (MOC) with formation of North Atlantic Deep Water (NADW) and the absence of a deep-water formation in the North Pacific is often considered to be caused by the fact that the North Pacific basin is a net precipitative, while the North Atlantic is a net evaporative basin. In this paper, the authors study the effect of asymmetries in continent geometry and freshwater fluxes on the MOC both in an idealized two-dimensional model and in a global ocean model. This study approaches the problem from a multiple equilibria perspective, where asymmetries in external factors constrain the existence of steady MOC patterns. Both this multiple equilibria perspective and the fact that a realistic global geometry is used add new aspects to the problem. In the global model, it is shown that the Atlantic forced by net precipitation can have a meridional overturning circulation with northern sinking and a sea surface salinity that resembles the present-day salinity field. The model results are suggestive of the importance of factors other than the freshwater flux asymmetries, in particular continental asymmetries, in producing the meridional overturning asymmetry.

scholarly journals Coupled Ocean–Atmosphere Responses to Recent Freshwater Flux Changes over the Kuroshio–Oyashio Extension Region

2011 ◽  
Vol 24 (5) ◽  
pp. 1507-1524 ◽  
Author(s):  
Liping Zhang ◽  
Lixin Wu ◽  
Jiaxu Zhang
Keyword(s):  
Coupled Model ◽  
Meridional Overturning Circulation ◽  
Western Boundary ◽  
Freshwater Flux ◽  
Boundary Current ◽  
The West ◽  
The North ◽  
Ocean Atmosphere ◽  
Set Up ◽  
The Kuroshio

Abstract Observations have indicated a trend of freshwater loss in the global western boundary current extension regions over several recent decades. In this paper, the coupled ocean–atmosphere response to the observed freshwater flux trend [defined as evaporation minus precipitation (EmP)] over the Kuroshio–Oyashio Extension (KOE) region is studied in a series of coupled model experiments. The model explicitly demonstrates that the positive EmP forcing in the KOE region can set up a cyclonic gyre straddling the subtropical and the subpolar gyre, which induces anomalous southward cold advection in the west and northward warm advection in the interior. This leads to the formation of a temperature dipole in the midlatitudes with a cooling in the west and a warming in the east. With the positive EmP forcing in the KOE, the response of the extratropical atmospheric circulation in the North Pacific sector is characterized by an equivalent barotropic low originating primarily from the western tropical Pacific changes and countered by the extratropical SST forcing. The positive EmP forcing also strengthens the tropical zonal SST gradient and thus ENSO through several competing processes including the surface-coupled wind–evaporative–SST (WES) mechanism, subduction of extratropical warm anomalies, and spinup of the density-driven meridional overturning circulation. Applications to recent Pacific climate changes are discussed.

Sign in / Sign up

Export Citation Format

Share Document